In an FTIR (Fourier Transform Infrared Spectroscope), infrared light from an infrared light source is split into two by a beam splitter, one of them is irradiated to a fixed mirror, the other is irradiated to a moving mirror, and these reflected light is guided to the same optical path again to be interfered. This interference light is irradiated to a sample which is a measurement target, and the transmitted light or the reflected light is detected and subjected to a Fourier transform to create a spectrum. From the peak wavelengths, the peak intensities, etc., of the spectrum, qualitative analyses and quantitative analyses of the sample are performed.
In the FTIR, normally, a control interference system is equipped as an optical system which is different from the aforementioned optical system of measurement light (infrared light) to be irradiated to a sample. The control interference system shares a beam splitter, a fixed mirror, and a moving mirror with the optical system of measurement light and is provided with a laser light source and a laser light detector for the control interference system. The control interference system generates interference light by passing laser light through the same interference system as the measurement light. Based on the interference signal of this laser light, the current position and the speed of the moving mirror are calculated.
The moving mirror is fixed to a movable unit capable of reciprocating on a guide, and the movable unit is driven by a driving motor. When driving force is applied from the driving motor to the movable unit, force which prevents a movement (movement resistance) due to the frictional force, the viscosity resistance of a lubricant, etc., is generated at the sliding portion of the movable unit and a guide portion. Therefore, by changing the voltage of the driving motor, the driving force to the movable unit is changed to adjust the moving speed.
As a control method of such a moving mirror speed, feedback control is sometimes used as described in Patent Document 1. In Patent Document 1, as shown in FIG. 6, the light from the laser light source 551 is interfered by the moving mirror 541 and the fixed mirror 544, and the intensity of the interference light is measured by the photodiode 552 to thereby calculate the current position and the speed of the moving mirror 541. Then, the control unit 530 applies the driving voltage to the driving motor 520 by feedback control by the PID control (Proportional-Integral-Derivative Control) based on, e.g., the deviation between the measured speed and the target speed, and performs the speed adjustment of the moving mirror 541.
In the case of measuring spectral characteristics of a sample, it is important to acquire accurate spectral data in order to perform accurate measurement. In the FTIR, an output signal is output every time the moving mirror passes through a specific position by the control interference system. During each output position, a detection signal of infrared interference light is accumulated in a detector and taken out as spectral data. Therefore, when the moving mirror is operated at a constant speed, the accumulation time every data becomes even, which enables stable acquisition of spectral data.